The motion picture industry is presently transitioning from traditional film based projectors to digital or electronic cinema. This trend is accelerating due to the popularity of 3-D movies. Even as digital cinema projection has matured and succeeded, largely based on the use of the well-known Digital Light Projection (DLP) technology, the promise of a further evolution to laser-based projection has been hovering in the background. Laser projection, whether for digital cinema, home projection, or other markets, has long been held back due to the cost and complexity of the laser sources, particularly in the green and blue spectral bands. As the necessary lasers are now becoming increasingly mature and cost competitive, the potential benefits expected from laser projection, including the larger color gamut, more vivid, saturated and brighter colors, high contrast, and low cost optics are increasingly being realized. An exemplary system is described in the paper “A Laser-Based Digital Cinema Projector”, by B. Silverstein et al. (SID Symposium Digest, Vol. 42, pp. 326-329, 2011).
Additionally, other commonly recognized laser projection problems, including the reduction of the visibility of laser speckle and the management of laser safety to reduce eye exposure risk or hazard potential, are increasingly being addressed in a satisfactory manner. As these issues are resolved, other less recognized issues will become increasingly important. As one example, image displays with narrow-bandwidth light sources, including lasers, can suffer from observer metameric failure, such that individual viewers can perceive significantly different colors.
In the field of color science, metamerism is the visual perception of color matching for color stimuli having different spectral power distributions. Two stimuli that have the same broadband spectral power distribution are said to be isomers, and will generally be seen as identical color matches by all observers. Whereas, two colors that appear identical to an observer but which have different spectral power distributions are called metamers. Due to differences in spectral sensitivity among observers, a metamer for one observer may not be a metamer for all observers. The greater the spectral differences between a pair of metameric stimuli, the more sensitive the color perception of that metameric pair will be to any changes in the illuminant, material compositions, observers, or field of view.
As can then be anticipated, there are various circumstances in which the spectral differences of a nominal metameric pair can lead to metameric failure, in which the expected color match is no longer perceived. As a first example, observer metameric failure (sometimes referred to as observer color perception variability) occurs when the color of two objects, or two elements in a displayed image, are perceived differently by two or more observers in the same viewing conditions. Observer metameric failure occurs because the optical system of the eyes, the color receptor response, and neural color processing varies among individuals. As another example, illuminant metameric failure occurs when colors match under one light source, but not another. For example, two color patches with different reflectance spectra can appear to be identical, providing the same color appearance when viewed under daylight illumination. However, then when viewed under fluorescent illumination, the color patches can appear differently, even to a single observer. Conditions of metameric failure are further defined to include field-size metameric failure and geometric metameric failure. Field-size metameric failure occurs because the relative proportions of the three cone types in the retina vary from the center to the periphery of the visual field. This is exemplified by the differences observed in the CIE standard observer color matching functions for 2° and 10° viewing fields (see FIG. 1B). Therefore, colors that match when viewed as very small, centrally fixated areas can appear different when presented over large color areas. Geometric metameric failure can occur when two samples match when viewed from one angle, but fail to match when viewed from a different angle. For the purposes of the present invention, observer metameric failure and illuminant metameric failure are of predominant interest.
One method to prevent metameric failure from occurring is to construct color imaging systems based on spectral color reproduction. Such systems would be based on the principle of isomers, with the relative spectral power distribution of scene color being carefully captured and then reproduced. A famous example is the Lippmann two-step method of photography introduced in 1891, in which color images were made and viewed essentially using spectral color reproduction, or regeneration of the captured scene's wavelength spectrum. However, such systems are complex, radiometrically inefficient, and sensitive to viewing angles. Therefore they have never come into wide use.
The fact that the human visual system has only three types of cone photoreceptors makes it possible for two stimuli to match in perceived color without having identical spectral power distributions, and thus metameric color matching occurs. In particular, each type of cone, red (long), green (medium), or blue (short), responds to the cumulative or integrated energy from a broad range of wavelengths. As a result, different combinations of light across all wavelengths can produce an equivalent receptor response. As long as the integrated responses of the three cone types are equal, for one spectrum compared to another, the stimuli will represent a metameric match, and will have the same perceived color to the observer.
Most practical color imaging systems use a limited set of colorants (typically three or four) and rely on the phenomenon of metamerism to produce color images having the desired color appearance, even though the reproduced color spectra will generally not match the original color spectra. The comparative amounts of the colorants provided by the color imaging system are adjusted to produce a color which will appear to closely match the original scene color. Modern color imaging systems are optimized to provide close color matches between the original and the reproduction for as many important colors as possible, relative to a standard observer, or set of observers.
As suggested previously, the phenomenon of metamerism, in which color matches occur despite spectral differences, is prone to failure. In general, metamerism depends on the interaction of the light source spectrum with the spectral reflectance/transmittance properties of the materials illuminated with the light, and the spectral response of the observer (or the camera sensor). Color perception among color normal observers varies depending on pre-retinal filtering in the optical media (cornea, lens, and humors), macular photo pigment density, cone distribution differences, color neural processing differences, and differences in cone spectral sensitivity. Human color perception can be measured using color matching functions (CMFs), which vary among individuals and are known to change with age. FIG. 1A illustrates twenty sets of color matching functions 300 measured for a set of different individuals having “normal” color vision, for 10° observational fields, using data from Table I (5.5.6) in the book “Color Science” by Wyszecki and Stiles (2nd Ed., John Wiley & Sons, New York, pp 817-822, 1982). In particular, FIG. 1A shows that color sensitivity can vary significantly among individual observers, with significant local variations of 5-10% or more at many wavelengths.
The Commission International de l'Eclairage (CIE) has documented color matching functions for two different standard observers: a 2° 1931 CIE standard observer and a 10° 1964 CIE standard observer. FIG. 1B compares the CIE 2° color matching functions 300a and CIE 10° color matching functions 300b. It is noted that the CIE 10° color matching functions 300b deviate from the CIE 2° color matching functions 300a in each of the red, green, and blue portions of color space, but the biggest differences occur in blue portion of color space. In particular, the largest differences occur in the blue (<500 nm), as the blue (short) color matching function response peaks ˜10% higher for the CIE 10° color matching functions 300b compared to the CIE 2° color matching functions 300a. Additionally, both the green (middle) and red (long) color matching functions crosstalk into the blue spectral range, and the color response differences between the respective CIE 2° color matching functions 300a and the CIE 10° color matching functions 300b are larger in the blue than in many portions of the red and green color spectra. In particular, above 540 nm, the blue color matching function lacks significant response with only two color matching functions (red and green) contributing significantly to the perception of color samples, the color differences between these cones being comparatively smaller. Also, the presence of the short wavelength “blue” cones is very small in the fovea and increases on the periphery. These differences reflect the fact that metameric color perception differences can be observed for different observational field sizes and can vary with wavelength.
Under normal circumstances, for example in daylight viewing conditions, the most common source of observer metameric failure is colorblindness (i.e., impaired color vision) among one or more observers. However, as the spectral properties of a light source or an object's reflectivity narrow and become more complex, and lack spectral color diversity, significant observer metameric failure can occur even amongst individuals who are considered to have normal color vision.
Systems (e.g., displays) that utilize narrow-band color primaries are most susceptible to observer metameric failure effects. Therefore, it can be anticipated that viewers of laser based displays, including digital laser projectors, and other displays with narrow spectrum primaries (such as LEDs), may experience observer metameric failure. While the expanded color gamut that laser displays can offer has been eagerly anticipated, in reality, it will include many wide-gamut colors that are not only outside of a conventional film or CRT display color gamut, but which are also seldom seen in nature. As a result, differences in color perception among observers of such very saturated colors at or near the gamut boundary may be hard to describe or quantify. On the other hand, color perception differences among observers for gamut colors of typical devices, and particularly for memory colors, such as sky blue, skin tones, or grass greens, which occur minimally with broadband light sources, can occur more frequently and dramatically with narrow-band light sources like lasers.
In the case of digital cinema, significant differences in color perception among expert observers viewing content involving memory colors in a color suite or screening room, may lead to significant dissatisfaction. For example, one expert observer may state that a displayed skin tone looks too green, while another may state that it looks too red. In such settings, problems can also emerge when comparing the narrow-band or laser displays to a broadband display that is an accepted standard. Moreover, even if a group of expert observers are satisfied, some members of a broader audience may not be, and such experiences may lead to dissatisfaction that may ultimately affect marketplace success of a narrow-bandwidth display technology.
Some approaches to mitigate the problem of observer metameric failure have been previously suggested or demonstrated. As an example, Thornton and Hale, in the paper “Color-imaging primaries and gamut as prescribed by the human visual system” (Proc. SPIE, Vol. 3963, pp. 28-35, 2000), consider the problem of observer metameric failure reduction for an additive color display systems having three narrow-band primaries (Δλ˜10 nm full-width-half-maximum (FWHM)). The authors proposed that to reduce the effects of observer metameric failure, the color primaries should preferentially be close to the so-called “prime wavelengths” (450, 540, and 610 nm) which are at or near the peaks of the three spectral sensitivities of the normal human visual system. Thornton et al. are not clear about how much improvement their method would be expected to yield. The Thornton prime wavelength primaries are overlaid on the color matching functions 300 in FIG. 1A as Thornton red laser primary 432, Thornton green laser primary 434 and Thornton blue laser primary 436. Also shown for comparison are a typical set of laser primaries for the exemplary laser projection system described in the aforementioned paper by Silverstein et al., including red laser primary 422, green laser primary 424 and blue laser primary 426.
In a more recent paper, “Minimizing observer metamerism in display systems,” by Ramanath, (Color Research and Application, Vol. 34, pp. 391-398, 2009), observer metameric failure for different types of displays having three primaries is examined. In particular, Ramanath explores the comparative occurrence of observer metameric failure among different electronic display devices, including CRT displays, LCD, DLP and LED based displays, a CCFL (cold cathode fluorescent lamp) based display, and a laser display. Ramanath concludes that observer metameric failure can occur more frequently, and provide greater perceived color differences, as the display spectrum narrows (smaller FWHM) or the number of modes in the display spectrum increases. As a result, the laser display and CCFL display, which lack spectral color diversity due to narrow or multi-modal spectra, have a high propensity to cause observer metameric failure. By comparison, the CRT and lamp based DLP displays, which have broad primaries (Δλ≈60-70 nm FWHM), exhibit low potential for observer metameric failure. In the case of laser displays, where the spectral bandwidths can easily be 2 nm or less in width, a small expansion of the lasing bandwidths, at the cost of a small color gamut decrease, would provide a reasonable trade-off if observer metameric failure is significantly decreased. However, Ramanath found that spectral distributions with moderate FWHM bandwidths (Δλ˜28 nm), such as LED illuminated displays, can still produce significant perceptible observer metameric failure, suggesting that reductions in observer metameric failure may not come quickly with increases in spectral bandwidth.
Ramanath also builds on the work of Thornton, and provides a modeled “ideal” set of primary spectral power distributions (SPDs) for a three primary display having primaries with spectral peaks close to the Thornton primaries, that may reduce the difference between the colors seen by a reference observer and a non-reference observer. In particular, Ramanath proposes that three broadband color channels or primaries, a blue primary with peak power at 450 nm and a bandwidth of Δλ˜49 nm, a green primary with peak power at 537 nm and a bandwidth of Δλ˜80 nm, and a red primary with peak power at 615 nm and a bandwidth of Δλ˜56 nm, will provide the least susceptibility to observer metameric failure. However, taken together, these two papers suggest that a three primary display having color spectra with preferential locations per Thornton, but moderate bandwidths (e.g., Δλ˜30 nm) will still exhibit significant metameric failure among observers. Thus, the guidance for minimizing observer metameric failure in a system with three narrow-bandwidth primaries is even less clear.
Other researchers have suggested that observer metamerism can be reduced by using more than three color primaries or color channels. In the paper “A multiprimary display: discounting observer metamerism” (Proc. SPIE, Vol. 4421, pp. 898-901, 2002), Konig et al. describe an image display system having six primaries that is used to display metamers and reduce observer metameric failure. This paper states that imaging systems having only three color signals as input (e.g. RGB values or L*a*b* values) cannot produce color reproductions that are precise for all human observers, as information is not available on how different observers perceive the original color. That is, the color vision response of the human visual system for an observer cannot be directly measured to determine how the primaries can be optimized. By comparison, the authors propose that a multiprimary display having more than three primaries introduces additional degrees of freedom for displaying a given color, such that perceptual color differences can be reduced for each observer. In particular, Konig et al. find that a multispectral display having more than three broadband primaries can provide both a large color gamut and the spectral control to reproduce color for each pixel by spectral color reproduction, such that observer metameric failure is minimized. An exemplary multi-spectral display is described, using two LCD projectors that provided overlapped images to a screen, that together provide an extended color gamut. This display has six broad bandwidth (Δλ˜40-100 nm FWHM) primaries, where one projector provides RGB images, and the second provides CMY images.
Fairchild and Wyble, in their paper “Mean observer metamerism and the selection of display primaries”, (Proc. 15th Color Imaging Conference, pp. 151-156, 2007), express concern about observer metameric failure occurring during the use of narrow-band primary displays, such as laser digital cinema projectors, causing consternation among filmmakers during “image proofing”. This paper models and compares differences in color perception for a display having broad bandwidth RGB primaries approximated by Gaussians having FWHM bandwidths of Δλ˜100 nm, and a second display with narrow-band primaries having FWHM bandwidths of Δλ˜5 nm, where the peak wavelengths of the color primaries were selected to be close to Thornton's prime wavelengths (450, 540, and 610 nm). After modeling age- and field-dependent color perception differences in terms of color matching functions (CMFs) and ΔE* color differences, Fairchild and Wyble conclude that color errors with displays having only three narrow-band primaries, such as laser projector, will be too large to be acceptable for critical color applications. The authors then suggest that display manufacturers, in developing displays capable of wider color gamut and greater luminance contrast, should abandon development of such narrow-band primary systems and redirect their efforts to systems that support spectral color reproduction. In particular, the authors suggest that emergent displays with large color gamut and enhanced luminance contrast should use multiple (N>3) wide-band primaries.
The paper “Display with arbitrary primary spectra” (SID Digest, Vol. 39, pp. 783-786, 2008), by Bergquist, provides an example of a multi-spectral display that attempts to reduce observer metameric failure. A field-sequential color display is described having a temporally-averaged, modulated array of N=20 Gaussian light sources (such as LEDs), each having FWHM bandwidths of Δλ˜30 nm, to approximate the spectrum of a color to be reproduced on the display. In this way, Bergquist provides a spectral reproduction system that synthesizes an approximation of the physical signal rather than emulating the sensation of color using superposition of a reduced set of narrowband primaries. Observer metameric failure is then reduced as compared to colorimetric matching, as a given observer would find the original scene and its reproduction to be identical (as the original and reproduced spectra are essentially identical). As a result, good agreement would be found among most observers, including many with color vision deficiencies, regardless of their interpretation of the sensation and the name they would give to a scene color. While the method is successful at reducing observer metameric failure, it requires many additional channels (N>>3) of color information at the capture stage (multispectral capture), and increased complexity in signal processing and display. None of this additional complexity is readily compatible with the image capture, processing, and display infrastructure of today or of the foreseeable future.
As another approach, Sarkar et al., in the paper “Toward reducing observer metamerism in industrial applications: colorimetric observer categories and observer classification” (Proc. 18th Color Imaging Conference, pp. 307-313, 2010), analyzed the Wyszecki and Stiles data and identified seven distinct groupings or categories of observers, for whom color vision, as measured by the respective CMFs, is statistically similar. With the goal of reducing observer metameric failure when using wide gamut displays, the authors suggest that a method for observer-dependent color imaging can be developed wherein the color workflow is tuned to match one of several observer classes. Of course, application of this method requires the classification of observers based on their color vision. While this approach might work for personalized color processing or small groups of people, it would not be extensible to helping the random assemblage of people present in a cinematic audience.
U.S. Pat. No. 6,816,284 to Hill et al., entitled “Multispectral color reproduction system with nonlinear coding,” provides a system that alters the color data captured by a multi-spectral camera using an encoding method that reduces the large amount of data required to represent the spectral information without causing a noticeable loss of the color information visible to an observer. As such, this patent is enabling data friendly spectral color reproduction as a means for reducing observer metameric failure when using N≧4 multi-primary displays.
Commonly assigned U.S. Pat. No. 7,362,336 to Miller et al., entitled “Four color digital cinema system with extended color gamut and copy protection,” discloses a multi-primary display having N≧4 narrow-band color channels that uses metameric matching to provide copy protection. In particular, it provides that selective rendering of portions of an image or image sequence can provide metameric matches by using the different combinations of primaries to provide the same color, but with varying spectral compositions, on a frame to frame basis. As such, the altered image portions can look similar to the human observers, but will look different to cameras that can be used to illicitly capture images from a projection or video screen. This approach exploits a special case of “observer” metameric failure that occurs between people and cameras to achieve the desired effect of copy protection, rather than reducing the occurrence of observer metameric failure among human observers.
In summary, while observer metameric failure has been identified as a problem that can affect display systems using narrow-band primaries, adequate solutions have not yet been suggested, particularly for displays having three primaries. The primaries used for the laser projection systems have narrow spectral bands. This leads to an increased color gamut and capacity to display highly saturated colors. At the same time, compared with existing reference displays, metameric mismatches are more frequent. Similarly, issues related to observer metameric failure in laser projection displays have also been documented.
The solutions offered to date are either incomplete, or require larger numbers of primaries (N>3) that preferentially have much wider spectra than lasers have, or require observer color matched displays. Thus, there remains a need for design approaches or operational methods that significantly reduce observer metameric failure for displays using narrow-band primaries without requiring observer-dependent color tuning or more than three primaries.